1. Field of the Invention
This invention relates to motion compensation for color video signals, and in particular to methods of and apparatus for such motion compensation such as in frame/field rate conversion or in signal compression/de-compression during video signal processing.
2. Description of the Prior Art
It is known in video processing to analyse the video image for stationary and moving parts of the image. This technique involves stationary parts of the image being repeated from frame to frame (or field to field) and the moving parts being defined by motion vectors mapping the movement of those parts. On the basis of such analysis, it is unnecessary to provide complete data for each separate frame of the video image; data relating to stationary parts of the image may be stored and then repeated, whereas data relating to moving parts may likewise be stored and repeated but with changing co-ordinates in successive frames/fields depending on the corresponding motion vectors. The motion compensation algorithms are generally based on inter-frame/field comparisons to select motion vectors for objects within the image.
One application of this technique is in image compression for transmission or storage. By repeating the stationary parts of the image at the same co-ordinates in successive frames/fields, and by repeating the moving parts of the image at co-ordinates changing in accordance with the motion vectors, a significant amount of data rate reduction can be achieved thereby compressing the video signal to be transmitted or stored. Upon reception or reproduction of the compressed video signal, the image can be reconstructed on the basis of the repeating co-ordinate information and the motion vectors.
Another application of this technique is in frame/field rate conversion when a video signal (for example, of one standard) at a particular frame/field rate needs to be converted into a video signal (for example, of another standard) at a different frame/field rate. It is then necessary to interpolate frames/fields between those defined by the original signal, and the use of motion compensation as described above is very effective in providing such interpolated images.
UK Patent Application Publication No. 2 231 228 A (the contents of which are incorporated herein by reference) discloses one way in which such motion vectors can be generated, and how data interpolation can be performed on the basis of the motion vectors.
Color video images, are represented by three orthogonal signal components, for example red (R), green (G) and blue (B), or combinations of these such as luminance (Y) and color difference components (C.sub.R, C.sub.B).
In order to obtain the optimum performance in motion compensation for color video signals, respective motion compensation algorithms should in theory be applied to all three components, this requiring individual application of the algorithm to each component and then combination of the three individual results. In practice, this involves an unacceptable usage of signal processing capability, and hence it is usual for only one component to be analysed. This results in a sub-optimal system being produced, the performance of which depends on the picture color content and the component which has been chosen. Typically, the luminance component is chosen as the single component for analysis. In this case, important information may be ignored in a brightly and distinctly colored scene. The luminance component is generally composed of 59% green, 30% red and 11% blue. Thus, in a luminance-only system, a predominantly blue scene (or, to a lesser extent, a predominantly red scene) will provide sub-optimal performance in view of the low proportion of the color contributing to the luminarice, leading to low levels of both the dynamic range of the input to the processor and also the signal-to-noise ratio.